As methods for synthesizing hydrocarbons from a syngas, the Fischer-Tropsch reaction (hereafter, referred to as an “FT reaction”), a methanol synthesis reaction and the like are well known. It is known that the FT reaction proceeds with a catalyst containing, as an active metal, an iron group element such as iron, cobalt and nickel, or a platinum group element such as ruthenium and the like; whereas the methanol synthesis reaction proceeds with a copper based catalyst; and the oxygen-containing C2 compound (such as ethanol and acetaldehyde) synthesis proceeds with a rhodium based catalyst (see, for example, Non-Patent Document 1).
Incidentally, in recent years, a diesel fuel of low sulfur content has been desired from the viewpoint of air environmental conservation, and it may be considered that this trend still more increases hereafter. Moreover, from the viewpoint that crude oil resources are limited or from the standpoint of energy security, it is desired to develop an alternative fuel, and it may be considered that this development is strongly desired more and more hereafter. As a technology responding to these desires, there is GTL (gas to liquids) which is a technology for synthesizing liquid fuels such as kerosene and gas oil and the like from a natural gas (main component: methane) whose proven reserves are said to be comparable to a crude oil in terms of energy. The natural gas does not contain a sulfur content; or even if it contains a sulfur content, the sulfur content is hydrogen sulfide (H2S) or the like which is easy for desulfurization, and therefore, the resulting liquid fuel such as kerosene and gas oil and the like does not substantially contain a sulfur content and possesses an advantage that it can be utilized as a high-performance diesel fuel having a high cetane number. Thus, this GTL has recently attracted attention more and more.
As a part of the foregoing GTL, a method for producing hydrocarbons from a syngas by the FT reaction has been actively investigated. The hydrocarbons obtained by this FT reaction include a wide range of hydrocarbons from methane to wax as well as a trace amount of oxygen-containing compounds such as various alcohols, and it is impossible to produce a certain specific fraction selectively. Thus, for example, in order to efficiently obtain fractions of kerosene and gas oil by the FT reaction, in addition to the fractions of kerosene and gas oil that are produced directly by the FT reaction, production of kerosene and gas oil fractions through hydrocracking of the wax fraction, which is heavier, to increase the yield of kerosene and gas oil fractions has been commonly performed.
On the other hand, the gasoline fraction can also be obtained by the FT reaction. However, because the hydrocarbons produced by the FT reaction are mainly composed of linear paraffins and olefins, the gasoline fraction obtained by the FT reaction has a very low octane number and no practical use. The octane number of hydrocarbons is usually the highest among aromatic hydrocarbons, followed by naphthenic hydrocarbons, olefinic hydrocarbons and paraffinic hydrocarbons in this order. Moreover, among the hydrocarbons of the same group, those with fewer carbon atoms, in other words, those with lower boiling points tend to have higher octane number, and those with more branches among those having the same number of carbon atoms tend to have higher octane number.
Conventionally, a method for producing a gasoline fraction through decomposition and isomerization of the hydrocarbons generated by the FT reaction using a solid acid catalyst such as zeolite has been proposed. For example, a method (one-step method) has been proposed for producing a gasoline fraction from syngas by one-step reaction using a catalyst that includes both an FT synthesis catalyst containing an FT active metal species (metal species that exhibits activity in the FT reaction), such as ruthenium and cobalt, and ZSM-5 or β-zeolite (see, for example, Patent Documents 1 and 2). In addition, a method (two-step method) has also been proposed for producing a gasoline fraction by a two-step reaction in which a reaction using an FT synthesis catalyst and a reaction using a solid acid catalyst are both conducted at the optimum temperatures (see, for example, Patent Documents 3).